Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most engines operate efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high-speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Automatic transmissions may include a torque converter for coupling a transmission input shaft to a crankshaft of the engine. To improve power-transfer efficiency, a controller may engage a bypass clutch to mechanically couple the transmission input shaft to the case of the torque converter, which is fixed to the flywheel of the engine. The bypass clutch may include one or more clutch plates that rotate with the case and are interleaved with one or more plates that rotate with the input shaft. To engage the clutch, pressurized fluid forces a piston to compress the clutch plates.
Most torque converters leak fluid to the transmission sump over time. This is referred to as torque converter drain-down. Systems and methods are disclosed herein for quickly filling the torque converter to minimize any negative effects of torque converter drain-down.